Astrophysics > Cosmology and Nongalactic Astrophysics

Title:
Lyman-alpha Forest Constraints on Decaying Dark Matter

Abstract: We present an analysis of high-resolution N-body simulations of decaying dark
matter cosmologies focusing on the statistical properties of the transmitted
Lyman-alpha forest flux in the high-redshift intergalactic medium. In this type
of model a dark matter particle decays into a slightly less massive stable dark
matter daughter particle and a comparably light particle. The small mass
splitting provides a non-relativistic kick velocity V_k to the daughter
particle resulting in free-streaming and subsequent damping of small-scale
density fluctuations. Current Lyman-alpha forest power spectrum measurements
probe comoving scales up to ~ 2-3 h^-1 Mpc at redshifts z ~ 2-4, providing one
of the most robust ways to probe cosmological density fluctuations on
relatively small scales. The suppression of structure growth due to the
free-streaming of dark matter daughter particles also has a significant impact
on the neutral hydrogen cloud distribution, which traces the underlying dark
matter distribution well at high redshift. We exploit Lyman-alpha forest power
spectrum measurements to constrain the amount of free-streaming of dark matter
in such models and thereby place limits on decaying dark matter based only on
the dynamics of cosmological perturbations without any assumptions about the
interactions of the decay products. We find that SDSS 1D Lyman-alpha forest
power spectrum data place a lifetime-dependent upper limit V_k < 30-70 km/s for
decay lifetimes < 10 Gyr. This is the most stringent model-independent bound on
invisible dark matter decays with small mass splittings. For large mass
splittings (large V_k), Lyman-alpha forest data restrict the dark matter
lifetime to Gamma^-1 > 40 Gyr. Forthcoming BOSS data should be able to provide
more stringent constraints on exotic dark matter, mainly because the larger
BOSS quasar spectrum sample will significantly reduce statistical errors.